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Electron-beam floating zone melting of refractory metals and alloys: Art and Science
,:.
,.
Int. J. of Refractory Metals & Hard Materials 12 (1993-1994) 295-301 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-4368/94/$7.00
,.
IIIgJl~. ! ' ~ ELSEVIER
Electron-Beam Floating Zone Melting of Refractory Metals and Alloys: Art and Science V. G. Glebovsky & V. N. Semenov Institute of Solid State Physics, 142432, Chernogolovka (Moscow region), Russia (Received 9 March 1994; accepted 17 May 1994) Abstract: Crucibleless methods are extremely important for melting, studying
and preparation of high purity refractory metals because of their high melting point and chemical reactivity in the liquid state. The electron-beam floating zone melting technique (EBFZM) was used both for purification of refractory metals and alloys from gaseous and metallic impurities and growing single crystals with predicted dislocation structure. Growing processes of bicrystals of refractory metals from the melt, with the grain boundary type and other bicrystalfine parameters determined in advance, are also studied. Bicrystals grown by EBFZM give the unique opportunity to study different grain boundary phenoma. Application of EBFZM to grow single crystal tubes of tungsten and molybdenum are discussed. Their real dislocation structure is studied. There is a possibility that high purity refractory metals prepared by EBFZM may be used as semiproducts for advanced metallurgical technology.
1 INTRODUCTION Single crystals of high-purity refractory metals (Nb,Ta,Mo,W, etc.) are widely used in modern material science and technology. This necessitates both investigation processes of purification and development of advanced methods of growing single crystals of high-purity refractory metals. The main purpose of this part of the project was to study the perfection of the real structure of single crystals of molybdenum and tungsten, as a function of parameters of the EBFZM technique such as the rate of crystallization, the rate of crystal rotation, the uniformity of heating, the fluctuation of the electron-beam intensity and the crystallographical perfection of the seed crystal. It was also of interest to study the specific features of the EBFZM growth of long single crystals, bicrystals and tubular single crystals of refractory metals. 2 EXPERIMENTAL To grow single crystals we used an electron-beam set-up (Fig. 1) developed in the Institute of Solid State Physics. ~ The set-up consists of a melting
Fig. 1. 295
The general view of the EBFZM set-up.
296
V. G. Glebovsky, V. N. Semenov
chamber, devices for crystal rotation and displacement of both an electron gun and a crystal, an electron gun, a vacuum pumping system and an electric power supply. The melting chamber consists of two parts, which provide a possibility to manage with both the gun and the sample. The pumping system provides a vacuum of 10- 5_10- 7 torr in the vacuum chamber. The total electric power input is about 25 kW with the operating voltage at 20 kV. The maximum length of the growing specimen is 1100 mm, with a diameter of 4-30 mm depending on the metal nature. The principal scheme of the melting chamber and the thermal zone of the EBFZM set-up is shown in Fig. 2. One can see the sample to be remelted, with the grown and seed crystals, the zone of liquid metal and the electron gun which can be displaced in the vertical direction at a rate varying from 0.5 mm min-~ to 50 nun min-~ depending on the nature and purity of metals to be melted. An original electron gun was elaborated on, for effective melting and crystal growth by EBFZM, because the gun is the most important element of the EBFZM setup. The principal scheme of the electron gun is presented in Fig. 3. The gun consists of a cathode, an anode and focusing elec-
trodes. The melting sample is an anode. Another part of the electron gun is the cathode, and it consists of a system of electrodes and a filament. The electrodes are made of copper, and water cooling is provided. The diameter of circular filament is about 55 ram. It is made of 1 mm diameter tungsten wire. The filament current is 40 A and the electric potential of the anode is + 20 kV. The electron gun provides the rotation of the electron beam and its focusing onto the sample. Generally speaking the electron gun is a system of electrostatic lenses. An arrangement of the electrodes of the electron gun makes it possible to vary the electron-beam-focusing from diffuse to sharp. The main advantage of a new electron gun is the possibility of using the device for a very long period of time (about 200 h) compared to common guns which cannot be used for longer than 20-30 min. Thus, this gun can be used continuously, both for the refining of metals and for the growing of single crystals, at growing rates of up to 50 ITllnmill-1 and diameters of samples up to 30 mm. Single crystal growth of refractory metals under the optimal vacuum conditions is accompanied by purification of liquid metal during the electron-beam zone melting. This was demonstrated by preparation of refractory metals with carbon and oxygen contents, which was sometimes lower in sensitivity than one of the physical and physico-chemical analytical methods used. 2 The following analytical techniques were used for analysis of gaseous impurities in metals: fast neutron activation, deutron activation and combustion methods. It has been established that a stage of so called 'diffusional transparency', when
4
Fig. 2. Thermal technological zone of the set-up: 1--cathode, 2--electrodes, 3--seed, 4--rod, 5--shields, 6--stems, 7--electric power supply, 8--ring clamps, 9--electron beam, 10--molten zone.
2
b
Fig. 3. The scheme of the electron gun: 1--specimen (anode), 2--upper accessory electrode, 3--cathode and circular electron beam, 4--focusing electrode, 5--liquid zone, 6--electron gun body, 7--lower accessory electrode.
297
E B F Z M of refractory metals and alloys." art and science
impurities diffuse very fast from the bulk to the surface of the melt, can be realized during vacuum electron-beam zone melting, owing to phase transitions of metal from solid to liquid state. 3,4
subgrains increases by no higher than an order of magnitude. During the growth from the melt, dislocations in crystals may arise because of seed structure inheritance, as a result of thermal stresses in a solid phase under the action of impurity concentration gradients due to lattice oversaturation with vacancies• As it can be seen from Fig. 6, the subgrains penetrate from the seed into the tungsten single crystal. To grow the crystal, the monocrystalline seed, free from small-angle boundaries, was used. The substructure-free seed was obtained by deformational annealing. The density of dislocations in the seed was 10 a cm-2. In this case we could expect the single crystal to have perfect crystallographic structure, but it appeared that dislocations inevitably arise again in the process of growth and their misorientation gradually increases. Two X-ray-divergent beam patterns
3 RESULTS AND DISCUSSION
3.1 Single crystals The dislocation structures of tungsten and molybdenum single crystals show a considerable similarity and vary insignificantly within the interval of displacement of the gun or crystal growing rates from 0"5 to 5 mm min-~.5 A typical substructure of the tungsten single crystal is shown in Fig. 4. The longitudinal sections exhibit subgrains elongated along the growth axis, the transverse groundends exhibit equiaxial subgrains but there is a radial inhomogenity of the structure. The subgrains get refined at the periphery. A noticeable change of the substructure occurs with increasing crystallization rate up to 10 mm min- 1 and higher. It involves the appearance of individual subgrains with misorientation up to 5 °. The same situation exists for growing single crystals of molybdenum. A typical size distribution of subgrains in single crystals of highly pure tungsten is shown in Fig. 5. The mean subgrain size is 400/~m. The experimental dependence of the dislocation density in subgrains versus the crystallization rate shows that at higher rates the dislocation density inside the
N 90
~0 70 60
2
~o +o
~ ~0
2O
Fig. 5. A typical subgrain size distribution: growth axis, 2--perpendicular.
~.:
--t ~-~'t..•;,,:/i .',~'"¢ t - ~:,.; , " :;¢'~.', " r'''
;.g4:.".
~
•
y...
~" ,--.-u,
.
or,
,
Fig. 4.
.'~
~'-!-~:~
-
f-:
~,~/..~
".
,
-'.~'.; .
•
'-
" ,
.
.
',
".~
.. . . . . _.'!" '[
::i' ;.L
- ¸'-.,
-,
. ';-'
:_;: ..... " \
Typical substructure of tungsten single crystals: Longitudinal and cross-section, respectively.
1--along
298
V. G. Glebovsky, V. N. Semenov
*g . v L•~
p.
Fig. 6.
Fig. 7.
.'~.# •
,
•
The appearance of fresh dislocation walls during W single crystal growth from the melt•
X-ray divergent beam patterns taken from the crystals grown from the melt and from the solid state, respectively (only Ka, lines are shown).
(Fig. 7) illustrate the difference in structures of the single crystals grown from the melt and those obtained by a solid-phase method• As much as the resistance ratio of the crystals in question was not worse than 104, we believe that impurities did not play an important role in the substructure formation. The high level of thermal stresses is probably responsible for the case. The EBFZM is characterized by the presence of significant temperature axial gradients in the solid phase and in the melt. We have estimated the value of these gradients by solving a onedimensional equation of thermal conductivity and by directly measuring the temperature, using an
optical micropyrometry method. In tungsten single crystals the temperature axial gradient under the crystallization front reaches 1500 K cm-~ and it gives rise to significant thermal stresses that are removed due to dislocations• As a result, the temperature gradient appears to be related to the density of nonremovable dislocations. The estimation coincides with the measured dislocation density in the crystals with better accuracy than a logarithmic one. We have also performed numerical estimations of the cooling rates of the crystals both in the process of growth at the stationary stage and after it has been terminated.
299
EBFZM of refractory metals and alloys." art and science
3.2 Bicrystals To carry out large-angle grain boundary studies in bicrystals we have developed a technique for growing refractory metal bicrystals using the EBFZM method, making it possible to obtain bicrystals with different planes of the boundary embedment and different misorientation angles of both special and general boundaries. 6 Figure 8 shows, schematically, three techniques for preparation of bicrystalline seeds. To grow bicrystals of molybdenum, niobium and tungsten up to 25 mm in diameter and 150 mm in length we used a modification of the EBFZM technique (Fig. 9). Molybdenum bicrystals with different crystallographic indexes of boundaries were tested for strength using three-point bending. It has been established that large-angle boundaries in molybdenum bicrystals exhibit a considerably lower strength than small-angle boundaries. A great number of the samples experienced brittle fracture along the grain boundary but some samples had insignificant regions of plastic deformation. Near a special tilt and twist boundary, a characteristic strength minimum was observed. The studies of the fracture surfaces by optical microscopy (Auger-spectroscopy) showed the presence of second-phase particles (maybe M%C) on the grain boundary. The Auger-spectrometry studies of cleavage planes revealed interstitial impurities, their concentration on the boundaries being one order of
magnitude greater than in the grain interior. The studies of the boundary strength of the molybdenum bicrystals have also shown that the strength of large-grain boundaries in the investigated range of impurity concentrations predominantly depends on the boundary type and the
Fig. 9.
z=3
[ io2 (Ifzj
The special issue is used to make bicrystal growth stable.
IlloJ J30i
f/t
/302
) t (ooIJ
EaoJ
I0011 -. 5 /?2
P
:) Fig. 8.
Three ways to prepare bicrystal seeds.
Fig. 10. Scheme for single crystal tube growth (stationary stage): 1--initial tube; 2--meniscus; 3--single crystal tube.
300
V. G. Glebovsky, V. N. Semenov
Fig. 11.
The angular scanning X-ray pattern taken from the cross-section of single crystal tube ((110) reflection).
magnitude of misorientation angle but not on the purity of the bicrystals. 3.3 Tubes
The EBFZM-grown single crystals are only of cylindrical form. For the purposes of new energy sources and for tube production, it is of interest to melt tubular ingots. The floating zone melting is, probably, the only possible method to both purify and solidify tube ingots of refractory metals from the melt. With this aim in view we have developed the technique for the EBFZM-growth of single crystals of W and Mo in the form of tubes from tubular samples obtained by a CVD method. 7,s After vacuum degassing, a molten meniscus is formed in the billet and the zone recrystallization is performed. In this case, tubular tungsten crystals up to 200 mm in length are obtained. The specific features of this method for growing tubular crystals at capillary shaping were studied. Schematic representation of the process at the stationary stage is shown in Fig. 10. Because the walls of the tube are thin ( 1 mm) it is possible to ignore the gravitational forces acting upon the meniscus and, in the thin-film approximation from the solution of the Laplace equation, to obtain the relationship between the billet and the crystal sizes. We have also studied the dislocation structure of the monocrystalline W tubes. In the case of seedless growth of a polycrystalline tube, large-grain boundaries locate along the growth axis. Small-angle boundaries in the cross-section of a monocrystalline tube behave analogously. Figure 11 demonstrates the X-ray angular scann-
ing pattern, taken from the cross-section of a monocrystalline tube with the growth axis (111) and the reflection (110). It is seen that misorientation angles between subgrains do not exceed 2 °. In their structure, the monocrystalline tubes and cylinders are identical.
4 CONCLUSIONS The electron-beam floating zone is an excellent technique for preparation of ingots of highly pure, refractory metals. Refractory metals and their alloys can be purified from gaseous impurities by the EBFZM technique only. No other techniques are known which can be used to purity refractory metals to low levels of impurities as can be done by the EBFZM. The technique is also the only one to produce such unique metallic products, for modern materials science and advanced industry, as single crystals, bicrystals and tubular crystals of principal crystallographic orientations. The modern set-ups for the EBFZM can be very effectively used for industrial applications.
REFERENCES 1. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Less-Common Metals, 117 (1986) 385-9. 2. Glebovsky, V. G. & Shipilevsky, B. M., J. Cryst. Grow., 60 (1982) 363-9. 3. Glebovsky, V. G., Kapchenko, I. V., Kireyko, V. V., Oblivantsev, A. M. & Rybasov, A. G., J. Cryst. Grow., 74 (1986) 529-35.
E B F Z M of refractory metals and alloys: art and science 4. Glebovsky, V. G., Shipilevsky, B. M., Kapchenko, I. V. & Kireyko, V. V., J. All. & Comp., 184 (1992) 297-304. 5. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Cryst. Grow., 87 (1988) 142-50. 6. Sursaeva, V. G., Glebovsky, V. G., Shulga, Yu.M. &
301
Shvindlerman, L. S., Scripta Metall., 19 (1985) 411-17. 7. Glevosky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Cryst. Grow., 98 (1989) 487-91. 8. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., Vacuum, 41 (1990) 2165-6.
,.
Int. J. of Refractory Metals & Hard Materials 12 (1993-1994) 295-301 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-4368/94/$7.00
,.
IIIgJl~. ! ' ~ ELSEVIER
Electron-Beam Floating Zone Melting of Refractory Metals and Alloys: Art and Science V. G. Glebovsky & V. N. Semenov Institute of Solid State Physics, 142432, Chernogolovka (Moscow region), Russia (Received 9 March 1994; accepted 17 May 1994) Abstract: Crucibleless methods are extremely important for melting, studying
and preparation of high purity refractory metals because of their high melting point and chemical reactivity in the liquid state. The electron-beam floating zone melting technique (EBFZM) was used both for purification of refractory metals and alloys from gaseous and metallic impurities and growing single crystals with predicted dislocation structure. Growing processes of bicrystals of refractory metals from the melt, with the grain boundary type and other bicrystalfine parameters determined in advance, are also studied. Bicrystals grown by EBFZM give the unique opportunity to study different grain boundary phenoma. Application of EBFZM to grow single crystal tubes of tungsten and molybdenum are discussed. Their real dislocation structure is studied. There is a possibility that high purity refractory metals prepared by EBFZM may be used as semiproducts for advanced metallurgical technology.
1 INTRODUCTION Single crystals of high-purity refractory metals (Nb,Ta,Mo,W, etc.) are widely used in modern material science and technology. This necessitates both investigation processes of purification and development of advanced methods of growing single crystals of high-purity refractory metals. The main purpose of this part of the project was to study the perfection of the real structure of single crystals of molybdenum and tungsten, as a function of parameters of the EBFZM technique such as the rate of crystallization, the rate of crystal rotation, the uniformity of heating, the fluctuation of the electron-beam intensity and the crystallographical perfection of the seed crystal. It was also of interest to study the specific features of the EBFZM growth of long single crystals, bicrystals and tubular single crystals of refractory metals. 2 EXPERIMENTAL To grow single crystals we used an electron-beam set-up (Fig. 1) developed in the Institute of Solid State Physics. ~ The set-up consists of a melting
Fig. 1. 295
The general view of the EBFZM set-up.
296
V. G. Glebovsky, V. N. Semenov
chamber, devices for crystal rotation and displacement of both an electron gun and a crystal, an electron gun, a vacuum pumping system and an electric power supply. The melting chamber consists of two parts, which provide a possibility to manage with both the gun and the sample. The pumping system provides a vacuum of 10- 5_10- 7 torr in the vacuum chamber. The total electric power input is about 25 kW with the operating voltage at 20 kV. The maximum length of the growing specimen is 1100 mm, with a diameter of 4-30 mm depending on the metal nature. The principal scheme of the melting chamber and the thermal zone of the EBFZM set-up is shown in Fig. 2. One can see the sample to be remelted, with the grown and seed crystals, the zone of liquid metal and the electron gun which can be displaced in the vertical direction at a rate varying from 0.5 mm min-~ to 50 nun min-~ depending on the nature and purity of metals to be melted. An original electron gun was elaborated on, for effective melting and crystal growth by EBFZM, because the gun is the most important element of the EBFZM setup. The principal scheme of the electron gun is presented in Fig. 3. The gun consists of a cathode, an anode and focusing elec-
trodes. The melting sample is an anode. Another part of the electron gun is the cathode, and it consists of a system of electrodes and a filament. The electrodes are made of copper, and water cooling is provided. The diameter of circular filament is about 55 ram. It is made of 1 mm diameter tungsten wire. The filament current is 40 A and the electric potential of the anode is + 20 kV. The electron gun provides the rotation of the electron beam and its focusing onto the sample. Generally speaking the electron gun is a system of electrostatic lenses. An arrangement of the electrodes of the electron gun makes it possible to vary the electron-beam-focusing from diffuse to sharp. The main advantage of a new electron gun is the possibility of using the device for a very long period of time (about 200 h) compared to common guns which cannot be used for longer than 20-30 min. Thus, this gun can be used continuously, both for the refining of metals and for the growing of single crystals, at growing rates of up to 50 ITllnmill-1 and diameters of samples up to 30 mm. Single crystal growth of refractory metals under the optimal vacuum conditions is accompanied by purification of liquid metal during the electron-beam zone melting. This was demonstrated by preparation of refractory metals with carbon and oxygen contents, which was sometimes lower in sensitivity than one of the physical and physico-chemical analytical methods used. 2 The following analytical techniques were used for analysis of gaseous impurities in metals: fast neutron activation, deutron activation and combustion methods. It has been established that a stage of so called 'diffusional transparency', when
4
Fig. 2. Thermal technological zone of the set-up: 1--cathode, 2--electrodes, 3--seed, 4--rod, 5--shields, 6--stems, 7--electric power supply, 8--ring clamps, 9--electron beam, 10--molten zone.
2
b
Fig. 3. The scheme of the electron gun: 1--specimen (anode), 2--upper accessory electrode, 3--cathode and circular electron beam, 4--focusing electrode, 5--liquid zone, 6--electron gun body, 7--lower accessory electrode.
297
E B F Z M of refractory metals and alloys." art and science
impurities diffuse very fast from the bulk to the surface of the melt, can be realized during vacuum electron-beam zone melting, owing to phase transitions of metal from solid to liquid state. 3,4
subgrains increases by no higher than an order of magnitude. During the growth from the melt, dislocations in crystals may arise because of seed structure inheritance, as a result of thermal stresses in a solid phase under the action of impurity concentration gradients due to lattice oversaturation with vacancies• As it can be seen from Fig. 6, the subgrains penetrate from the seed into the tungsten single crystal. To grow the crystal, the monocrystalline seed, free from small-angle boundaries, was used. The substructure-free seed was obtained by deformational annealing. The density of dislocations in the seed was 10 a cm-2. In this case we could expect the single crystal to have perfect crystallographic structure, but it appeared that dislocations inevitably arise again in the process of growth and their misorientation gradually increases. Two X-ray-divergent beam patterns
3 RESULTS AND DISCUSSION
3.1 Single crystals The dislocation structures of tungsten and molybdenum single crystals show a considerable similarity and vary insignificantly within the interval of displacement of the gun or crystal growing rates from 0"5 to 5 mm min-~.5 A typical substructure of the tungsten single crystal is shown in Fig. 4. The longitudinal sections exhibit subgrains elongated along the growth axis, the transverse groundends exhibit equiaxial subgrains but there is a radial inhomogenity of the structure. The subgrains get refined at the periphery. A noticeable change of the substructure occurs with increasing crystallization rate up to 10 mm min- 1 and higher. It involves the appearance of individual subgrains with misorientation up to 5 °. The same situation exists for growing single crystals of molybdenum. A typical size distribution of subgrains in single crystals of highly pure tungsten is shown in Fig. 5. The mean subgrain size is 400/~m. The experimental dependence of the dislocation density in subgrains versus the crystallization rate shows that at higher rates the dislocation density inside the
N 90
~0 70 60
2
~o +o
~ ~0
2O
Fig. 5. A typical subgrain size distribution: growth axis, 2--perpendicular.
~.:
--t ~-~'t..•;,,:/i .',~'"¢ t - ~:,.; , " :;¢'~.', " r'''
;.g4:.".
~
•
y...
~" ,--.-u,
.
or,
,
Fig. 4.
.'~
~'-!-~:~
-
f-:
~,~/..~
".
,
-'.~'.; .
•
'-
" ,
.
.
',
".~
.. . . . . _.'!" '[
::i' ;.L
- ¸'-.,
-,
. ';-'
:_;: ..... " \
Typical substructure of tungsten single crystals: Longitudinal and cross-section, respectively.
1--along
298
V. G. Glebovsky, V. N. Semenov
*g . v L•~
p.
Fig. 6.
Fig. 7.
.'~.# •
,
•
The appearance of fresh dislocation walls during W single crystal growth from the melt•
X-ray divergent beam patterns taken from the crystals grown from the melt and from the solid state, respectively (only Ka, lines are shown).
(Fig. 7) illustrate the difference in structures of the single crystals grown from the melt and those obtained by a solid-phase method• As much as the resistance ratio of the crystals in question was not worse than 104, we believe that impurities did not play an important role in the substructure formation. The high level of thermal stresses is probably responsible for the case. The EBFZM is characterized by the presence of significant temperature axial gradients in the solid phase and in the melt. We have estimated the value of these gradients by solving a onedimensional equation of thermal conductivity and by directly measuring the temperature, using an
optical micropyrometry method. In tungsten single crystals the temperature axial gradient under the crystallization front reaches 1500 K cm-~ and it gives rise to significant thermal stresses that are removed due to dislocations• As a result, the temperature gradient appears to be related to the density of nonremovable dislocations. The estimation coincides with the measured dislocation density in the crystals with better accuracy than a logarithmic one. We have also performed numerical estimations of the cooling rates of the crystals both in the process of growth at the stationary stage and after it has been terminated.
299
EBFZM of refractory metals and alloys." art and science
3.2 Bicrystals To carry out large-angle grain boundary studies in bicrystals we have developed a technique for growing refractory metal bicrystals using the EBFZM method, making it possible to obtain bicrystals with different planes of the boundary embedment and different misorientation angles of both special and general boundaries. 6 Figure 8 shows, schematically, three techniques for preparation of bicrystalline seeds. To grow bicrystals of molybdenum, niobium and tungsten up to 25 mm in diameter and 150 mm in length we used a modification of the EBFZM technique (Fig. 9). Molybdenum bicrystals with different crystallographic indexes of boundaries were tested for strength using three-point bending. It has been established that large-angle boundaries in molybdenum bicrystals exhibit a considerably lower strength than small-angle boundaries. A great number of the samples experienced brittle fracture along the grain boundary but some samples had insignificant regions of plastic deformation. Near a special tilt and twist boundary, a characteristic strength minimum was observed. The studies of the fracture surfaces by optical microscopy (Auger-spectroscopy) showed the presence of second-phase particles (maybe M%C) on the grain boundary. The Auger-spectrometry studies of cleavage planes revealed interstitial impurities, their concentration on the boundaries being one order of
magnitude greater than in the grain interior. The studies of the boundary strength of the molybdenum bicrystals have also shown that the strength of large-grain boundaries in the investigated range of impurity concentrations predominantly depends on the boundary type and the
Fig. 9.
z=3
[ io2 (Ifzj
The special issue is used to make bicrystal growth stable.
IlloJ J30i
f/t
/302
) t (ooIJ
EaoJ
I0011 -. 5 /?2
P
:) Fig. 8.
Three ways to prepare bicrystal seeds.
Fig. 10. Scheme for single crystal tube growth (stationary stage): 1--initial tube; 2--meniscus; 3--single crystal tube.
300
V. G. Glebovsky, V. N. Semenov
Fig. 11.
The angular scanning X-ray pattern taken from the cross-section of single crystal tube ((110) reflection).
magnitude of misorientation angle but not on the purity of the bicrystals. 3.3 Tubes
The EBFZM-grown single crystals are only of cylindrical form. For the purposes of new energy sources and for tube production, it is of interest to melt tubular ingots. The floating zone melting is, probably, the only possible method to both purify and solidify tube ingots of refractory metals from the melt. With this aim in view we have developed the technique for the EBFZM-growth of single crystals of W and Mo in the form of tubes from tubular samples obtained by a CVD method. 7,s After vacuum degassing, a molten meniscus is formed in the billet and the zone recrystallization is performed. In this case, tubular tungsten crystals up to 200 mm in length are obtained. The specific features of this method for growing tubular crystals at capillary shaping were studied. Schematic representation of the process at the stationary stage is shown in Fig. 10. Because the walls of the tube are thin ( 1 mm) it is possible to ignore the gravitational forces acting upon the meniscus and, in the thin-film approximation from the solution of the Laplace equation, to obtain the relationship between the billet and the crystal sizes. We have also studied the dislocation structure of the monocrystalline W tubes. In the case of seedless growth of a polycrystalline tube, large-grain boundaries locate along the growth axis. Small-angle boundaries in the cross-section of a monocrystalline tube behave analogously. Figure 11 demonstrates the X-ray angular scann-
ing pattern, taken from the cross-section of a monocrystalline tube with the growth axis (111) and the reflection (110). It is seen that misorientation angles between subgrains do not exceed 2 °. In their structure, the monocrystalline tubes and cylinders are identical.
4 CONCLUSIONS The electron-beam floating zone is an excellent technique for preparation of ingots of highly pure, refractory metals. Refractory metals and their alloys can be purified from gaseous impurities by the EBFZM technique only. No other techniques are known which can be used to purity refractory metals to low levels of impurities as can be done by the EBFZM. The technique is also the only one to produce such unique metallic products, for modern materials science and advanced industry, as single crystals, bicrystals and tubular crystals of principal crystallographic orientations. The modern set-ups for the EBFZM can be very effectively used for industrial applications.
REFERENCES 1. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Less-Common Metals, 117 (1986) 385-9. 2. Glebovsky, V. G. & Shipilevsky, B. M., J. Cryst. Grow., 60 (1982) 363-9. 3. Glebovsky, V. G., Kapchenko, I. V., Kireyko, V. V., Oblivantsev, A. M. & Rybasov, A. G., J. Cryst. Grow., 74 (1986) 529-35.
E B F Z M of refractory metals and alloys: art and science 4. Glebovsky, V. G., Shipilevsky, B. M., Kapchenko, I. V. & Kireyko, V. V., J. All. & Comp., 184 (1992) 297-304. 5. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Cryst. Grow., 87 (1988) 142-50. 6. Sursaeva, V. G., Glebovsky, V. G., Shulga, Yu.M. &
301
Shvindlerman, L. S., Scripta Metall., 19 (1985) 411-17. 7. Glevosky, V. G., Semenov, V. N. & Lomeyko, V. V., J. Cryst. Grow., 98 (1989) 487-91. 8. Glebovsky, V. G., Semenov, V. N. & Lomeyko, V. V., Vacuum, 41 (1990) 2165-6.